1. Field of Invention
The present invention relates to a drive circuit. More particularly, the present invention relates to a drive circuit for driving light emitting diodes (LEDs).
2. Description of Related Art
The most important issues in today are about energy saving and carbon emission reduction and the Greenhouse effect. A traditional bulb generates 0.683 Kg of CO2 emissions when one kilowatt-hour of electrical power is consumed by this bulb. Typically, 525 kilowatt-hours of electrical power are consumed by a bulb in one year, which will generate more carbon and increase the Greenhouse effect. Therefore, the trend today is to replace the bulbs with light emitting diodes.
A light-emitting diode (LED) is a semiconductor light source. When a light-emitting diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is usually small in area (less than 1 mm2), and integrated optical components shape its radiation pattern and assist in reflection. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. Light-emitting diodes are used in applications as diverse as replacements for aviation lighting, automotive lighting (particularly indicators) and in traffic signals. The current flows through LED determine its brightness.
FIG. 1 illustrates a typical drive circuit for a LED. As shown in the FIG. 1, a voltage source 100 provides a current I to the LEDs 104, 106, 107, 108 through the resistor 102. If the resistance of the resistor 102 is R and the resistance of each LED 104, 106, . . . , 108 is RD, the current I flowing through LEDs 104, 106, . . . , 108 will be
  I  =            V              R        +                  n          ×                      R            n                                .  
However, the typical drive circuit can not supply a fixed output current. That is, the current I is changeable based on the temperature. The increasing temperature of the LEDs 104, 106, . . . , 108 will increase the current I even though a fixed voltage is supplied to the LEDs 104, 106, . . . , 108. For example, when a LED is powered for a long time, the temperature of this LED increases. Then, the increasing temperature will change the current I following through the LEDs 104, 106, . . . , 108 even though the voltage supplied to the LEDs 104, 106, . . . , 108 is not changed. In other words, the brightness of the LEDs 104, 106, . . . , 108 will be changed based on the temperature. On the other hand, when a voltage supplied to the typical drive circuit is changed, the output current is also changed in the typical drive circuit. Accordingly, the typical drive circuit can not supply a stable output current to the LEDs to illuminate a stable brightness, which limits the development of the LEDs used in a light system.
Therefore, a drive circuit that can supply a stable output current to the LEDs is needed.